1. Field of the Invention
The present invention relates to a tracking error detection method applicable commonly to an optical disk medium for different recording systems, and a diffraction grating and a photodetector for use in the above detection method.
2. Description of the Related Art
In general, in prior art optical disk recording systems, when information is read out after being recorded, due to the fact that an apparatus in the system detects errors such as when a laser beam spot cannot trace a normal track or a groove, there is a conventional tracking error detection method which employs error detecting spots before and after each main spot for recording and reading in addition to the main spots. But since this method has the disadvantage of not being applicable to a phase variation type optical disk or an additional storage type optical disk, it has long been desired to develop more advanced tracking error detection system technology which can be utilized for the numerous types of optical disks in common use.
In general, a microspot of a laser beam projected by an optical head is irradiated onto a predetermined track of a rotary optical disk recording medium to effect recording and reproducing of data. The laser beam spot must always remain on the track. Since the track is often displaced due to an inclination of an optical path based on the rotation of the disk, it is necessary to have the laser beam spot follow the displacement. The follow means consists of detecting a shift of the irradiation position of the spot and mechanically moving an objective lens used for the laser beam. The detection of the shift of the spot irradiation position is referred to as detection of a tracking error.
In a prior art, a push-pull method or a three-beam method has been employed as tracking error detection methods, but neither of them is completely satisfactory.